The invention relates generally to the preparation of hydrophobic inorganic oxide-based pigments, and more specifically to methods for treating inorganic oxide pigments in aqueous media with organosilicon compounds in order to introduce hydrophobic characteristics to the pigment.
Titanium dioxide (TiO2) is the premier white pigment used for whitening, brightening and opacifying paper, paints and plastics. Like many other inorganic oxides, untreated TiO2 pigment is hydrophilic in nature and, as such, is readily wetted by water but not wet by nonpolar substances, such as hydrocarbons, plastics, etc. Accordingly, in order to incorporate TiO2 into plastics, oil-based paints and other nonpolar substances, the surface of the TiO2 must often be made hydrophobic to improve the dispersibility of the pigment in the nonpolar substance and to improve other properties of the resulting pigmented substance.
Numerous agents have been developed for making TiO2 hydrophobic, including phosphorylated fatty acid derivatives, dialkylsulfosuccinates and various silicone-based agents. Some examples of xe2x80x9csilicone-based agentxe2x80x9d include silanes, substituted silanes, hydrosylates and condensates of these silanes, siloxanes, polysiloxanes, and substituted siloxanes and polysiloxanes. The substituted groups are often organic and/or functional, i.e. chemically reactive, groups. For the purposes of this disclosure this class of substituted silanes, siloxanes and polysiloxanes agents will be referred to as xe2x80x9corganosiliconxe2x80x9d agents.
In one approach for preparing hydrophobic TiO2 pigment, the TiO2 particles are treated with chemically nonreactive organosilicon compounds, such as polydimethylsiloxanes or other such agents. Typically, such agents are applied to inorganic oxides, like pigments, by spraying the agents undiluted or in solution onto the particle surfaces. However, the resulting pigments typically exhibit poor physical properties for high temperature applications and the nonreactive agent can migrate from the surface of the pigment leading to potential problems with heat sealing, print clarity and other such related problems in the final pigmented product.
Another approach for preparing hydrophobic TiO2 pigment involves treating the inorganic pigment with a reactive silane that will bond directly to the pigment surface. U. S. Pat. Nos. 5,607,994; 5,631,310; 5,889,090; and 5,959,004 (all assigned to E. I. du Pont de Nemours and Company; herein referred to collectively as the xe2x80x9cDuPont Patentsxe2x80x9d) teach the coating of TiO2 pigment with octyltriethoxysilane where it is intended that the silane directly bonds to the pigment surface. Typically, the reactive silane, for example an octyltriethoxysilane in the DuPont Patents, is sprayed directly onto the pigment when the pigment is dry or incorporated in a filter cake. Some prior art methods use mechanical mixers to facilitate the dispersing of the silane onto the pigment.
The disadvantages of these prior art techniques include incomplete coating of the pigment particles by the reactive silane, even with mechanical mixing, and less than optimal binding of the reactive silane to the pigment particle surface. Additionally, such techniques also produced processing problems with respect to the manufacture of the pigment and often introduce environmental problems by generating volatile organic compounds, such as ethanol.
Other prior art methods include applying reactive silanes in an organic slurry in order to get more complete and uniform coating of the pigment particles. The disadvantages of these techniques is that usually the organic solvent must be removed before processing of the pigment can proceed; thus adding an additional and expensive processing step to the production of the pigment.
The inventors in their earlier patent, U.S. Pat. No. 5,653,794, the disclosure of which is hereby incorporated by reference, describe a technique which overcomes the disadvantages of using organic solvents and the prior art environmental problems with respect to the generation of volatile organic compounds. This patent describes a treatment method for TiO2 pigment wherein a halosilane is added to an aqueous slurry of TiO2 pigment and water. The use of a halosilane, such as octyltrichlorosilane, avoids generation of volatile organic compounds, such as ethanol. In this process, hydrochloric acid is generated in solution as the by-product of the hydrolysis reaction of the silanes. Since the hydrochloric acid is in solution, it can easily be neutralized and disposed of as a salt; thus avoiding environmental problems associated with the release of volatile organic compounds. Moreover, because the reaction takes place in an aqueous slurry of pigment and water this treatment procedure does not require the removal of organic solvents from the pigment in order to continue processing.
It has been found, however, that the addition of the halosilane to an aqueous solution of pigment and water results in a high viscosity mixture which is often difficult to process. The present invention overcomes this difficulty while retaining the advantages of U.S. Pat. No. 5,653,794.
The present invention provides a treatment method for rendering inorganic oxide particles, such as TiO2 pigment, hydrophobic which avoids the disadvantages of the prior art. It has been found that by predispersing a suitably reactive organohalosilane into an aqueous media and using intensive mixing means (such as a rotor stator emulsifier or inline static mixer), a reactive dispersion can be prepared. When this dispersion is combined with inorganic oxide particles, such as TiO2 pigment, under sufficient agitation, the problems discussed above of the prior art are eliminated. The present inventive method provides unexpected processing advantages in pigment preparation and yields a more economical pigment product. Pigment preparation time and the processing equipment energy requirements are reduced. Additionally, the hydrophobic pigments prepared by using the present invention have good performance properties, such as dispersibility in nonpolar substances (i.e., plastics), and they do not degrade physical properties of the pigmented nonpolar substance, such as lacing resistance of thermoplastics.
The present section details the preferred embodiments of the invention. These embodiments are set forth to illustrate the invention and aid in its understanding. Since this disclosure is not a primer on inorganic oxides or TiO2 pigment production or their treatment or the design, manufacture or operation of treatment vessels, basic concepts known or readily determinable by those skilled in the field of TiO2 production have not been set forth in detail. Concepts such as choosing appropriate manufacturing materials or solvents, or appropriate additives for the treatment process, or suitable conditions for operating the treatment process are known or readily determinable by those skilled in the industry. Attention is directed to the appropriate texts and references known to those in the art field for details regarding these and other concepts that may be necessary in the practice of the invention.
Methods for treating TiO2 pigment with organosilicon compounds are well known in the prior art and are discussed in numerous references, including U. S. Pat. Nos. 2,488,440; 2,559,638; 4,061,503; and 4,151,154, the DuPont Patents and U.S. Pat. No. 5,653,794 by the present inventors. The teachings of these patents are incorporated into the present disclosure by reference.
The subject invention provides a method for preparing a hydrophobic inorganic oxide by (i) mixing a reactive silane with an aqueous media under conditions so that the reactive silane substantially condenses and forms an aqueous reactive dispersion and (ii) mixing the resulting aqueous dispersion with a slurry of pigment and water under conditions so that the reactive dispersion chemically reacts with the inorganic oxide directly or upon subsequent heating or drying.
Suitable reactive silanes useful in the practice of the invention maybe represented by the formula:
RnSiX4xe2x88x92n
where R represents a nonhydrolyzable aliphatic, cycloaliphatic or aromatic group (including a saturated or unsaturated branched or unbranched alkyl, aryl cycloalkyl or alkylaryl group) having 1 to 20 carbon atoms, X represents a halogen and n=1 or 2. Preferably, the reactive silane is an organochlorosilane, such as butyltrichlorosilane, isobutyltrichlorosilane, hexyltrichlorosilane, octyltrichlorosilane, octylmethyldichlorosilane, decyltrichlorosilane, dodecyltrichlorosilane, didecyldichlorosilane, dihexyldichlorosilane, dioctyldichlorosilane, octadecyltrichlorosilane or mixtures thereof.
More preferably, the reactive silane is an organochlorosilane having an R group with 4-10 carbons and, in particular, 6-8 carbons. The most preferred organohalosilane is hexyltrichlorosilane.
Inorganic oxides appropriate for use in the present invention include those which have surface hydroxyls capable of condensing with reactive silanes or their hydrolyates or condensates. Such inorganic oxides are represented by the rutile and anatase forms of titanium dioxide, kaolin and other clays, alumina, silica, aluminum trihydrate, zirconium oxide, zinc oxide, iron oxide, cadmium pigments, chromate pigments, chromium oxide pigments, glass fibers, glass flake, wollastonite and the like. Preferred are standard pigment-grade titanium dioxides, regardless of whether they have been obtained from titanium tetrachloride or from titanium sulfate.
The inorganic oxide can be an untreated inorganic oxide or an inorganic oxide whose surface has been treated by deposition thereon of phosphate, alumina, silica, zirconia and the like, using procedures known to practitioners in the field of inorganic oxide surface treatment. Preferably, the inorganic oxide is titania or TiO2 pigment, whose surface is untreated or treated by deposition thereon of a hydrous phosphate, alumina, silica, zirconia or like-material.
A preferred mode for practicing the claimed invention typically proceeds as follows. A reactive organohalosilane, such as hexyltrichlorosilane (HTCS), is mixed with an aqueous media using a commercial type static mixer or emulsifier to form a dispersion that contains substantially the condensates of the silane. It is preferable to mix the silane with the aqueous media using a high speed emulsifier or agitator, such as an in-line rotor-stator mixer, in order to keep the resulting condensate well dispersed and to minimize any unreacted silane. Although agitation is not critical, the agitation should be sufficient to avoid gelling of the silane, its condensates or the dispersion. The residence time of the dispersion in the mixer typcally is very short, 0.5 to 1 second, although longer times may be used. The dispersion is aged (usually in the mixer or in a process line or batch mixer downstream) for about 1 second to about 120 seconds, preferably for around 20 seconds. Longer aging times can be used if sufficient agitation is present.
It is also preferred to maintain a pH value above or equal to about nine to aid in driving the conversion of the silane toward condensates of the silane. Although a high pH value of around 9 is preferred, it is contemplated that the invention may be practiced at lower pH values also, even pH values of 1-2. At lower pH values, there may be less conversion of the silane to its condensates but a reactive dispersion would still be formed. It is also contemplated that the invention will be practiced at the pH value that results from mixing the silane with the aqueous media; that is, no adjustment of the pH is made. It is also contemplated that the dispersion that results from mixing the silane with the aqueous media may be heated to aid in conversion of the silane to its condensates. The temperature to which the dispersion may be heated will be determined, for the most part, by the type of equipment that is available. Typically the dispersion may be heated to 70-90xc2x0 C.
The resulting aqueous dispersion is added over a period of time to a reactor containing a slurry of TiO2 pigment and water. The pigment slurry should be sufficiently agitated such: that the reactive dispersion is rapidly mixed into the bulk of the pigment slurry. The amount of aqueous dispersion added to the reactor is such so as to result in a concentration of organosilicon coating on the final pigment in the range of about 0.1 to about 5.0 weight percent based on the weight of the pigment.
After the dispersion is mixed with the pigment slurry, the entire mass is allowed to come to an equilibrium pH and consequently a uniform viscosity. The pH is then raised to flocculate the organically treated pigment and to facilitate filtration and washing of the pigment. Typically, additions of an alkali source, such as sodium hydroxide, are used to raise the pH up to a value of about 7-10 pH (although lower pH values may be used, including values as low as about 4). The slurry is aged for about 30 to 60 minutes to assure a uniform mixture. Additional adjustments may be made to the pH during or after aging. The pigment is then separated, washed until the oxide has a electrical resistivity of at least 3000 ohms, dewatered, sufficiently dried, collected and micronized.
Following micronizing, the treated pigment can be used for dispersion into a plastic material, such as a polyolefin material, typically polyethylene. The pigments of the present invention have shown excellent dispersion properties in plastics and do not degrade the lacing resistance properties of the resulting pigmented plastic films.
The hydrophobic pigment products of this invention offer processibility in polymer composites as least as good as pigments prepared according to prior art procedures; that is, the dispersibility of the products of the present invention in polymers is at least as good as prior art pigments and the lacing resistance of the resulting polymer composites containing the products of the present invention is as good as prior art polymer composites.
Representative polymers in which the products of the present invention may be used include, but are not limited to, polymers of ethylenically unsubstituted monomers, including polyethylene, polypropylene, polybutylene and copolymers of ethylene with alpha-olefins containing 4 to 12 carbon atoms or vinyl acetate; vinyl homopolymers, acrylic homopolymers and copolymers, polyamides, polycarbonates, polystyrenes, acrylonitrile-butadiene-styrenes, polyethers and the like.